In the past, as the material for fuel tanks, Pb—Sn-alloy-coated steel sheet superior in corrosion resistance, workability, solderability (weldability), etc. has mainly been used. This has been broadly used for automobile fuel tanks. On the other hand, Sn—Zn-alloy-coated steel sheet, for example as shown in Japanese Unexamined Patent Publication (Kokai) No. 52-130438, has mainly been produced by electroplating involving electrolysis in an aqueous solution including Zn and Sn ions. Sn—Zn-alloy-coated steel sheet having Sn as a main ingredient is superior in corrosion resistance and solderability and has been made much use of for electronic components etc.
Further, Sn-coated steel sheet is being made wide use of mainly for food can and beverage can applications due to the superior corrosion resistance and workability of Sn. However, it is known that while Sn has a sacrificial corrosion protection ability for the base iron in an environment free of dissolved oxygen such as the inside of a food can, it has the defect of easily progression of corrosion from the base iron in environments with oxygen present. As technology for making up for this, the technology of applying steel sheet coated with Sn—Zn alloy containing 20 to 40% Zn to electronic components, auto parts, and other after coating fields is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-116794. However, this is by electroplating. In electroplating of Sn, the current density is low, so a high amount of deposition has been difficult to obtain for reasons of cost and productivity.
It was discovered that this Sn—Zn-alloy-coated steel sheet has superior properties in automobile fuel tank applications. Japanese Unexamined Patent Publication (Kokai) No. 8-269733 and Japanese Unexamined Patent Publication (Kokai) No. 8-269734 disclose hot-dip Sn—Zn-alloy-coated steel sheet.
The above-mentioned Pb—Sn alloy-coated steel sheet used as the material for automobile fuel tanks has been recognized as having various superior properties (for example, workability, corrosion resistance at the inside surface of the tank, solderability, seamless weldability, etc.) and has been favored in use, but along with the recent rising awareness of the global environment, a shift is occurring in the direction of Pb-free materials. On the other hand, Sn—Zn-alloy electroplated steel sheet has mainly been used for electronic components where solderability etc. are required, i.e., applications where the corrosive environment is not that severe.
Further, hot-dip Sn—Zn-alloy-coated steel sheet indeed has superior corrosion resistance, workability, and solderability. However, in recent years, further improvement of the corrosion resistance has been sought. In Sn—Zn-coated steel sheet, pitting due to Zn segregation sometimes occurs even at flat parts not subjected to any working, but in salt water spray tests envisioning salt corrosive environments, the time until occurrence of red rust is short, so the corrosion resistance in a salt corrosive environment cannot be said to be sufficient. To further improve the sacrificial corrosion protection ability, it is sufficient to increase the amount of addition of Zn, but if the amount of Zn is too high, the coating layer shifts from mainly Sn to Zn and the dissolution of Zn itself is far greater than Sn, so the corrosion resistance of the coating layer itself is impaired. Further, this hot-dip Sn—Zn-alloy-coated steel sheet has an alloy layer including at least one of Fe, Zn, and Sn. This alloy layer grows continuously thick. An alloy layer is a reaction product between the coating metal and the base iron and forms an intermetallic compound layer. Therefore, in general, it is a brittle layer. If grown thick, working will cause fractures leading to lamellar peeling at the inside. From this sense, a hot-dip Sn—Zn-alloy-coated steel sheet having a continuous thick alloy layer tended to be somewhat inferior in workability.
Further, Sn—Zn-alloy coated steel sheet having a thick alloy layer has a tendency for segregation of the Zn at the Sn—Zn solidified structure. This is because on a continuous homogeneous alloy layer, there are few nuclei for coating solidification, so a coarse solidified structure results. In a coarse solidified structure, segregation of Zn easily occurs, so an Sn—Zn-alloy-coated steel sheet tends to be somewhat inferior in corrosion resistance.